Charge-coupled device (CCD) image sensors capture images through photosensitive sites (“pixels”) that generate charge in response to the intensity of incident light. The pixels are generally arranged in rows and columns to form a pixel array. To read the accumulated charges out of the pixel array, a vertical CCD shift register is either positioned adjacent to or contained within each column of pixels to receive and shift the accumulated charges to a horizontal CCD shift register. The charges are shifted through the vertical CCD shift register in a row-by-row or in-parallel process. The horizontal CCD shift register receives the charges from the vertical CCD shift registers and sequentially shifts each row of accumulated charges to an output amplifier serially or a pixel-by-pixel.
Each pixel has a maximum amount of charge that it can store. A phenomenon known as “blooming” occurs when the total number of charge carriers collected by a pixel exceeds the charge capacity for that pixel and the excess charge spills over into adjacent pixels. Many CCD pixels are formed with an anti-blooming structure, such as a lateral overflow drain. These structures provide a way to drain the excess charge carriers from the pixel before the charge carriers spill into adjacent pixels.
In conventional quad-output full frame CCD image sensors, a center gate along a horizontal center line of the image sensor may transmit light and generate charge. The charge may drift towards the neighboring pixels in the rows above and/or below the center gate resulting in image artifacts along the rows above and below the center line since the charge that drifts to neighboring pixels is not evenly distributed. The extra charge collected in the neighboring pixels creates an image artifact manifesting itself as two brighter lines along the center rows with different intensities compared to their neighboring rows above and below.